Group III-V compounds are important semiconductor materials useful for fabricating optic components (particularly, short wavelength LEDs and lasers) and electronic components (particularly, high-temperature/high-power transistors). However, the commercial exploitation of these III-V compounds has been hindered by the practical impossibility of directly growing bulk single crystal substrates of these substances. As most of these materials are therefore manufactured by heteroepitaxial CVD processes, their commercial exploitation has also been hindered by lack of practical epitaxial substrates for growth of low-defect density, monocrystalline layers of III-V compounds.
In the case of III-nitrides, one state-of-the-art approach to improving the quality of heteroepitaxially-grown material is to grow very thick layers. It has been discovered that, even though the lower portions of a thick layer may have excessive defect densities, the upper portions can nevertheless have adequate defect densities. However, growth of thick layers requires growth rates currently achievable only with hydride (halide) vapor phase expitaxy (HVPE). HVPE processes are conducted in CVD reactors, use precursor gases including corrosive chlorine compounds, and are conducted with a substrate at significantly elevated internal temperatures so as to promote a desired reaction between the precursor gases.
However, not all internal reactor components need be maintained at the high reaction/deposition temperatures, nor is it desirable to do so. For example, the walls of the CVD reactor chamber can be maintained at lower temperatures in order to limit unwanted deposition of target material on the reactor walls. Also, certain components within the reactor, e.g., o-rings, seals, gaskets, and the like, should be maintained at lower temperatures because they are often made of polymeric materials which can be damaged by higher temperatures.
On the other hand, it is also not desirable for internal components to be at too low a temperature. For example, precursor gases or reaction products can condense on internal components at lower temperatures. Also, internal components at lower temperatures can cause undesirable interactions, such as complex formation, between the gases present in the reactor. Such effects can damage internal components and cause contamination of a growing semiconductor wafer.
Certain components can be both exposed to the interior of a reactor chamber and include temperature-sensitive materials, e.g., polymeric seals, and such components can be subject to the conflicting requirements, being at both higher and lower temperatures. Examples of such components (with conflicting temperature requirements) present in many commercial CVD reactors are gate valves (or load locks or transfer doors, or the like). Such commercial reactors often feature automated transfer of wafers between the interior of the reactor chamber and an intermediate environment known as a transfer chamber (or load chamber or transfer chamber, or the like). Such automated transfer means usually include an automated gate valve which, when open, accommodate wafer transfer and the automated wafer transfer devices, and which, when closed, seal the interior of the chamber from the exterior.
Briefly, when closed, at least a face of a gate valve is exposed to the reactor-chamber interior, and the temperature of the exposed face should be high enough so that described undesirable condensation/reaction is limited. Also, when closed, the face of a gate valve should form a seal with an external face of the reactor, and the temperature in the vicinity of this must be low enough so that damage to sealing materials, e.g., gaskets, o-rings, and the like, is limited. Preferably, any thermal damage to the sealing materials is sufficiently low so that the gate valve can be opened and closed many thousands times without repair.
In the particular case of HVPE growth of GaN, a gate valve (or the isolation valve) assembly can be exposed to process gases including GaCl3, NH3, HCl, N2, H2 and Ar and to reaction products and by-products including NH4Cl. Of these, the more problematic are the precursor gas GaCl3 and the reaction product NH4Cl. GaCl3 is corrosive and can condense on surfaces below ≈80° C. NH4Cl can condense on any surface inside the reaction chamber which is less than 150° C. Further, only a limited number of sealing materials are sufficiently resistant to corrosive chlorine-containing gases, and these generally cannot withstand temperatures in excess of ≈150-160° C.
The prior art describes methods purporting to protect reactor components from corrosive materials and to prevent condensation on cooler reactor components. For example, U.S. Pat. No. 6,071,375 teaches protecting an internal component in a reactor by arranging it in a recessed pre-chamber that is open to the reactor but kept relatively free of process gases by an inert purge gas stream injected into the pre-chamber. However, such arrangements involving pre-chambers would be cumbersome for high volume manufacturing equipment which generally has stringent size limitations. Also, U.S. Pat. No. 6,086,673 teaches a two zone, hot-wall reactor. In a precursor production zone, the temperature is of the order of 850° C., whilst in a growth zone, the temperature is of the order of 1050° C. However, hot wall reactors are inappropriate for high volume manufacturing processes because nitride deposition on the hot walls necessitates frequent chamber cleaning. Also, the problem of sealing an automated gate valve is not addressed here. U.S. Pat. No. 5,156,521 illustrates an exemplary prior art gate valve.
The prior art is lacking in sealing methods and gate valve apparatus for a CVD chamber suitable for use in high volume manufacture of semiconductor materials, especially high volume manufacture of III-nitride and GaN materials.